Re: How difficult would it be to fix a British Election Count? i.e. Amber Rudd's Count.
Careful there AC, those are the sort of allegations that get one sued into a hole in the ground if false, and lets just say that if they were true, one probably shouldn't mention them in public.
On a more practical level, there's twenty polling stations in Hastings and Rye (info here), and they're counted by (paid) volunteers who are watched carefully, and the entire counting process is designed to be transparent and audit-able. You can read the recommendations and regulations here.
Giving the instructions a quick read, I can't see any easy way to influence the count, without controlling several people, so no, it would be very tricky to rig a UK election, especially without any time for preparation.
In advance, the easiest thing to do would be to invent false voters, and have someone go round and place their votes, but again, this would be tricky.
It's also worth noting, that although you conflate MI5, and GCHQ and assume they could and would conspire, the employees of both are civil servants, at least some (hopefully most) of whom would fight against any attempt to subvert the democratic process, just on principal.
TL/DR It would be more difficult than just paying someone to smear the opposition candidate ahead of time.